The invention is related to an optical waveguide device and method for creating an asymmetrical optical filter device. In particular the invention is related to an optical waveguide device with a special structure of core and inner and outer cladding so that an asymmetrical coupling of fundamental mode with the cladding modes of the device can be obtained.
The invention is also related to a method for creating an asymmetrical optical filter device by a special coupling of the fundamental mode with multiple cladding modes using special fiber design.
Bragg gratings in optical fibers have been known to be extremely useful features in the field of telecommunication for quite some years. Versatile filter and reflector systems can be carried out conveniently by use of such gratings. An important application is the optical gain equalization in long distance fiber lines. In order to compensate for the loss over long distances optical amplifiers are incorporated into the lines in certain spatial intervals. Unfortunately, in multiplexed systems the amplifier gain response usually is not flat over the whole range of all wavelength channels. Thus, a gain equalization by spatially succeeding filter systems is necessary. The filter characteristics, i.e. the “shape” of the filter, has to be optimized according to the characteristics of the fiber line, the amplifier systems and the wavelength range.
Slanted Bragg gratings SBG are used to couple light of certain over amplified wavelengths into the cladding modes of the fiber, while other wavelengths remain unaffected. The SBG is a standard fiber Bragg grating tilted during photo inscription with an angle between the grating fringes and the normal of the fiber axis. This SBG couples part of the fiber-guided mode into radiation modes or cladding modes in a counter-propagating direction. The envelope of couplings into the different cladding modes yields the filter shape. The envelope of couplings is defined by the specific fiber design. It is for example known from FR-A-9806904, to provide a photosensitive cladding to reduce the spectral width of the filter and a reduced photosensitivity in the core to decrease the back reflection into the fundamental mode, whereby the grating can be short (800 μm) or chirped to suppress the modulation due to coupling into discrete modes. By chirping the fringe period along the grating length, or by shortening the grating, each discrete filter corresponding to the coupling into each cladding mode is enlarged, so that they can overlap each other.
Especially in systems of a wide spectral range or in systems with very uneven spectral distribution, however, a single slanted Bragg grating often is not enough to yield a satisfying equalization of the amplifier gain. Thus, a chain of several slanted Bragg gratings with different characteristics can be concatenated one after the other to represent a row of spectrally succeeding filters. The succeeding fiber sections containing one filter each are spliced together.
In the U.S. Pat. No. 6,005,999 an optical filter is disclosed that offers a narrowband rejection filter that couples out of a forward propagating core mode. The design of this device allows the coupling of the fundamental mode to one cladding mode. The coupling to a single cladding mode allows to design the narrow band filters with for example a FWHM of around 0.3 nm. The filter characteristic is symmetric.
For a filter with a broader spectral width more than one cladding mode is coupled to the fundamental mode LP01. The French application 98 06 904 disclosed a filter having a symmetric shape, defined by the envelope of the cladding modes in the fiber. Often the gain equalization in transmission systems requires asymmetric shapes of fillers. Up to now solutions with symmetric concatenated filters are used.